Since the origination of echo cancellation as described in the article “An adaptive Echo Canceller” Sondhi in The Bell System Technical Journal, Mar. 1967 and U.S. Pat. Nos. 3,499,999 to Sondhi and 3,500,000 to Kelly et al., it has been known that the maximum amount of echo that can be removed is inherently limited by numerous immutable factors, such as quantization employed in telephone networks, despite the developmental efforts by others such as described in U.S. Pat. No. 3,789,165 to Campanella et al. The remaining echo, often termed residual echo, is nonetheless perceptible to a participant in a telephone call where the overall path delay of the connection is excessive. Hence, means are further employed on echo cancellers to eliminate the residual echo. These means are generically referred to as non-linear processors and may also be referred to as suppressors or center clippers. The method of center clipping, which removes all signals below a certain energy level is described for example in U.S. Pat. No. 4,031,338 to Campanella et al. assigned to Comsat.
Non linear processing, which removes residual echo, often creates some new problems, the most serious of which is alteration of the background signal. Specifically, when a non-linear processor removes residual echo, it usually removes the background signal as well, often replacing it with low level noise, often referred to as comfort noise, so that the participants on the call do not experience pure silence which can be perceived as a dead phone line. Often the original background signal was not noise at all, but music or the sounds of people in a crowd. Such non linear processing can have the effect, therefore, of interfering with the character of the background, which, in some cases, may be important to the character of the call.
Referring to FIG. 2, a communication path consists of two end paths 21 and 22 and a long haul or bulk delay path 23. Typically, echo heard at a near end 21 is due to echo generated by the telephone 25 at the far end 22. Hence the delay involved in the perception of echo is twice the total of each end-path delay combined with the bulk delay.
FIG. 2 illustrates a representative communication link 20 between two telephones 24 and 25. The link is comprised of a near-end 21, a far-end 22, and a communication network 23 that interconnects the near-end 21 and far-end 22. The near-end 21 has a user telephone 24, a hybrid circuit 26, and an echo canceller circuit 28. Similarly, the far-end 22 has a user telephone 25, a hybrid circuit 27, and an echo canceller circuit 29. Far-end signal power, x, is received by the near-end. Signal y is the coupled echo signal from the far-end signal as well as the near-end signal produced by telephone 24. This near-end signal contains both the speech of the near-end telephone user and the background noise of the user's environment. Together, the near-end signal and far-end echo signal are represented as y.
As an illustrative example, a telephone call in Los Angeles may be routed through an Echo canceller in San Francisco, over a long haul circuit to another echo canceller in Chicago, and then to a terminating phone in Detroit. If the end-path delay between Los Angeles and San Francisco is 8 ms, the long haul delay from San Francisco to Chicago is 24 ms, the end-path delay from Chicago top Detroit is 6 ms, The total end-to-end delay is then 38 ms. The delay of the echo, termed round trip delay, is then twice the end-to-end delay or 76 ms. However, a call from Boston to Washington, which terminates locally at each end will have a very small end path delay and a bulk delay of 9 ms, hence a round trip delay of only 18 ms.
It is well known that the perception of echo is both a function of the level of the echo and the delay of the echo. In fact, echoes with delays of less than 10 ms are not perceived as echo at all. However, what is not generally appreciated is that low level echoes are imperceptible at significantly longer delays. In the two examples above, deployment of non-linear processing for the calls from Los Angeles to Detroit would improve the quality of the call. On the other hand, deployment of a non-linear processors the second example, from Boston to Washington, would degrade the call.
FIG. 1 illustrates a simplified block diagram of an equipment configuration for one terminal of a communication link which includes a near end hybrid. The communication link has a near-end 8 comprising a telephone 2, a four-to-two wire hybrid circuit 3, an echo canceller circuit 4, a filter 5 and a NLP 6. A far-end connected to communication network 23, can be similarly configured but is not illustrated in FIG. 1. During a conversation between the near-end and far-end users, the far end signal, x, which contains both the far-end user's speech and background noise, enters the near-end 8 as signal x at node 9.
The far-end signal x is provided to the four-to-two wire hybrid circuit 3 and then to near-end telephone 2. Due to the unavoidable non-linearities present in the hybrid circuit 3, some portion of the far-end signal power is coupled onto the output 7 of the hybrid circuit 3 as an echo. A composite signal y exists at node 7 containing the echo signal and the combined speech of the near-end user and any incidental background noise from the near-end user's environment. A filter having a filter length period selected and designed to be longer than the hybrid dispersion time is used prior to power level measurements at 7 to allow the echo canceller 4 to operate properly.
Echo canceller 4 synthesizes the expected value e of the echo signal from adaptive filter 5, and subtracts this value at 10 from the composite signal y existing at node 7. The resulting difference signal existing at node 14, is intended to contain only the near-end signal s originating from telephone 2. This signal may be further processed by the NLP 6 and is ultimately provided to the far-end telephone as signal z, through the communications network 23.
Echo is an important factor in communications which include a hybrid between a four wire communication network 23 and the end terminals 24 and 25 as illustrated in FIG. 2. When echo is present, and perceptible, it is preferable to eliminate the echo. When the echo is not perceptible, it may be preferable to avoid echo elimination and the resultant signal distortion. Typical efforts to eliminate echo required that the magnitude of the echo be determined. One way of determining the magnitude of the echo is through echo return loss (ERL) estimation.
Methods of measuring the echo return loss typically measure the power of a signal x at node 9, where the signal power from the far-end would normally exist. Additionally, the power level of the composite signal y, comprised of the coupled echo signal and any signal s generated by the near-end telephone 2, is measured at node 7. The measurement can be made when little-to-no signal is being generated at near end telephone 2. Assuming the signal power of any signal generated by the near-end telephone is very small in comparison to the coupled echo signal power, the ratio of the measured test signal power x to the measured power level y provides an estimate of the echo return loss (ERL) for the near-end 8. The magnitude of echo return loss is usually measured as a difference in dB between signal x and signal y.
A typical echo canceller, as illustrated in FIG. 1, includes an adaptive finite impulse filter FIR 5. Under the control of an adaptation algorithm, FIR filter 5 models the impulse response of the echo path. A non-linear processor (NLP) 6 can be used to remove residual echo that may remain after linear processing of the input signal. The echo canceller may also typically include a double talk detector 11. Double talk occurs when both far end and near end speech are present at the same time. A double talk detector 11 can also be used to control and inhibit the adaptation process of the FIR 5 and/or the NLP 6 when double talk is present and it may be undesirable to cancel or suppress echo because double talk will be suppressed.
In the echo canceller, the signal y is the perceived near end signal. Signal y is a combination of the actual near end signal s and the echo from the far end signal x which comes through hybrid 3. The output signal d from the combiner 10, is the signal y less the echo estimate generated by the adaptive filter 5. The adaptive filter 5 is programmed to generate an output signal e that is as close to the echo as possible so that the echo is largely cancelled by the echo estimate e and the difference signal d closely resembles the generated near end signal s. The NLP 6 controls the amount of signal d that is transmitted to the far end. When there is no near end signal s, or a large echo over riding near end signal is present, NLP 6 can provide comfort noise to the far end instead of near end signal so as to prevent any possible uncancelled echo from being transmitted. When a valid s exists, NLP 6 opens so as to let the far end hear the signal. False detection of a lack of near end signal s can cause clipping of speech and failure to detect echo can result in echo leak through the NLP. The NLP as an on/off switch can result in abrupt audible changes which are undesirable in speech communications. Further, the use of the NLP 6 when the echo would be imperceptible to participants in the call causes unneeded distortion of the signal.
Referring to FIG. 1, typically the non-linear processor 6 is co-located with the echo canceller circuit 4. Usually an echo canceller 4 is aware of the end path delay in the end path 8 that it is canceling. However, a canceller typically has no knowledge of the bulk delay or of the end path delay at the far end. For this reason, a canceller 4 cannot normally effect a control on an associated non-linear processor 6 based upon bulk path delay. The common practice therefore is to leave non-linear processors enabled for all telephone calls. Hence the best possible quality may not be obtained on shorter connections.